Semiconductor device, silicon wafer and silicon ingot

ABSTRACT

A CZ silicon ingot is doped with donors and acceptors and includes an axial gradient of doping concentration of the donors and of the acceptors. An electrically active net doping concentration, which is based on a difference between the doping concentrations of the donors and acceptors varies by less than 60% for at least 40% of an axial length of the CZ silicon ingot due to partial compensation of at least 20% of the doping concentration of the donors by the acceptors.

PRIORITY CLAIM

This application claims priority to German Patent Application No. 102014 107 590.0 filed on 28 May 2014, the content of said applicationincorporated herein by reference in its entirety.

BACKGROUND

In silicon devices such as insulated gate bipolar transistors (IGBTs),diodes, insulated gate field effect transistors (IGFETs), for examplemetal oxide semiconductor field effect transistors (MOSFETs) a number ofrequirements need to be met. Such requirements typically depend uponspecific application conditions. Typically, trade-offs between linkedcharacteristics such as, for example high electrical breakdown voltageand low on-state resistance have to be found. Avalanche breakdown eventsand undesired formation of inversion channels, for example at silicon tooxide interfaces that may occur during operation of the semiconductordevice may have a negative impact on device robustness and devicereliability.

As a typical base material for manufacturing a variety of suchsemiconductor devices, silicon wafers grown by the Czochralski (CZ)method, e.g. by the standard CZ method or by the magnetic CZ (MCZ)method or by the Continuous CZ (CCZ) method are used. In the Czochralskimethod, silicon is heated in a crucible to the melting point of siliconat around 1416° C. to produce a melt of silicon. A small silicon seedcrystal is brought in contact with the melt. Molten silicon freezes onthe silicon seed crystal. By slowly pulling the silicon seed crystalaway from the melt, a crystalline silicon ingot is grown with a diameterin the range of one or several 100 mm and a length in the range of ameter or more. In the MCZ method, additionally an external magneticfield is applied to reduce an oxygen contamination level.

Growing of silicon with defined doping by the Czochralski method iscomplicated by segregation effects. The segregation coefficient of adopant material characterizes the relation between the concentration ofthe dopant material in the growing crystal and that of the melt.Typically, dopant materials have segregation coefficients lower than onemeaning that the solubility of the dopant material in the melt is largerthan in the solid. This typically leads to an increase of dopingconcentration in the ingot with increasing distance from the seedcrystal.

It is desirable to improve robustness and reliability of siliconsemiconductor devices. It is further desirable to provide a siliconingot and a wafer as a base material for such silicon semiconductordevices.

SUMMARY

An embodiment of a semiconductor device includes a silicon semiconductorbody comprising a drift zone of net n-type doping. An n-type doping ispartially compensated by 20% to 80% with p-type dopants. A net n-typedoping concentration in the drift zone is in a range from 1×10¹³ cm⁻³ to1×10¹⁵ cm⁻³.

A CZ silicon ingot according to an embodiment is doped with donors andacceptors and includes an axial gradient of doping concentration of thedonors and of the acceptors. An electrically active net dopingconcentration, which is based on a difference between the dopingconcentrations of the donors and acceptors varies by less than 60% forat least 40% of an axial length of the CZ silicon ingot due to partialcompensation of at least 20% of the doping concentration of the donorsby the acceptors.

According to another embodiment, a silicon wafer comprises a net n-typedoping. An n-type doping is partially compensated by 20% to 80% withp-type dopants. The net n-type doping concentration is in a range from1×10¹³ cm⁻³ to 1×10¹⁵ cm⁻³.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present disclosure and together with the description serve toexplain principles of the disclosure. Other embodiments and intendedadvantages will be readily appreciated as they become better understoodby reference to the following detailed description.

FIG. 1 illustrates a schematic cross-sectional view of a silicon waferaccording to an embodiment.

FIG. 2A illustrates a schematic cross-sectional view of a verticalsemiconductor device according to an embodiment.

FIG. 2B illustrates a schematic cross-sectional view of a lateralsemiconductor device according to an embodiment.

FIG. 3 illustrates a schematic cross-sectional view of a powersemiconductor diode according to an embodiment.

FIG. 4 illustrates a schematic cross-sectional view of a powersemiconductor IGBT according to an embodiment.

FIG. 5 is a schematic process chart illustrating a method ofmanufacturing a silicon wafer.

FIG. 6 is a schematic cross-sectional view of a CZ growth system forcarrying out the method illustrated in FIG. 5.

FIG. 7 is a schematic cross-sectional view of a crucible forillustrating a method of doping the crucible with dopant material.

FIG. 8 is a schematic cross-sectional view of a part of a CZ growthsystem for illustrating a method of adding dopants to a silicon melt inthe crucible.

FIG. 9 is a graph illustrating a simulated concentration ofnon-compensated phosphorus along an axial position of a CZ grown siliconingot with respect to different ratios of boron and phosphorus added tothe silicon melt.

FIG. 10 is a graph illustrating a simulated specific resistance along anaxial position of a CZ grown silicon ingot with respect to differentratios of boron and phosphorus added to the silicon melt.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which are shownby way of illustrations specific embodiments in which the disclosure maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described for one embodiment can be used on or inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present disclosure includes such modifications andvariations. The examples are described using specific language thatshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only. Forclarity, the same elements have been designated by correspondingreferences in the different drawings if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open and the terms indicate the presence of stated structures,elements or features but not preclude the presence of additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may exist between the electrically coupled elements, forexample elements that temporarily provide a low-ohmic connection in afirst state and a high-ohmic electric decoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n⁻” means adoping concentration that is lower than the doping concentration of an“n”-doping region while an “n⁺”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

FIG. 1 refers to a schematic cross-sectional view of a silicon wafer 100according to an embodiment.

The silicon wafer 100 comprises a net n-type doping. The net n-typedoping is illustrated in the schematic graph by a curve c₁ related to anet n-type doping concentration profile along a vertical direction ybetween opposite first and second surfaces 101, 102 of the silicon wafer100. In the illustrated example, the curve c₁ corresponds to thedifference of curve c₂ being a profile of n-type doping along thevertical direction y and curve c₃ being a profile of p-type doping alongthe vertical direction y. The n-type doping is partially compensated by20% to 80% with p-type dopants. With regard to the illustrated example,curve c₃ may range between a lower concentration limit c_(L)corresponding to 20% of c₂ and an upper concentration limit c_(H)corresponding to 80% of c₂.

Each of the curves c₂ and c₃ may slightly deviate from a constant leveldue to, for example segregation effects during Czochralski crystalgrowth. Due to different segregation coefficients for curves c₁, c₂,also curve c₃ may slightly deviate from a constant level, for exampleshow a small gradient in a vertical direction.

According to an embodiment, the n-type doping comprises phosphoruspartially compensated by boron as the p-type doping.

According to another embodiment, the net n-type doping is furthercompensated by a p-type dopant species having a segregation coefficientsmaller than phosphorus. According to yet another embodiment, the netn-type doping is further compensated by a plurality of different p-typedopant species having segregation coefficients smaller than phosphorus.By carrying out partial compensation by boron having a segregationcoefficient greater than phosphorus and by one or more p-type dopantspecies having segregation coefficients smaller than phosphorus, aneffective segregation of p-type dopants during CZ growth can be adaptedto the segregation behavior of phosphorus. Gallium (Ga) and aluminum(Al) are examples of p-type dopant species having a segregationcoefficient smaller than phosphorus.

The silicon wafer 100 allows for semiconductor devices having improvedrobustness and reliability. For example, avalanche breakdown at highblocking voltages may be diminished due to a reduced carrier mobilitycaused by the p- and n-type dopants. Furthermore, phosphorus pile-upeffects and boron pile-down effects at semiconductor to oxideinterfaces, for example in an edge termination area and/or in trenchesmay result in an enhanced total n-type doping at the semiconductor/oxideinterface allowing for a reduction of undesired inversion channelformation during device operation.

FIG. 2A is a schematic cross-sectional view of a portion of a verticalsemiconductor device 2001 according to an embodiment. The verticalsemiconductor device 2001 comprises a silicon semiconductor body 204.The silicon semiconductor body 204 may correspond to the silicon wafer100 illustrated in FIG. 1 or may be part of the silicon wafer 100, forexample a die resulting from wafer dicing. The vertical semiconductordevice 2001 includes a drift zone 205 of net n-type doping. An n-typedoping in the drift zone 205 is partially compensated by 20% to 80% withp-type dopants. The partially compensated drift zone 205 may correspondto a basic doping of a chip substrate material such as the silicon wafer100 illustrated in FIG. 1. The resulting n-type drift zone doping c₁ maybe between 1×10¹³ cm⁻³ and 1×10¹⁵ cm⁻³, or between 2×10¹³ cm⁻³ and2×10¹⁴ cm⁻³, or between 3×10¹³ cm⁻³ and 7×10¹³ cm ⁻³.

The vertical semiconductor device 2001 includes a first load terminalstructure 220 at a first surface 210, e.g. front surface of thesemiconductor body 204. The first load terminal structure 220 includesdoped semiconductor region(s). The doped semiconductor region(s) may beformed by doping processes of the silicon semiconductor body 204 at thefirst surface 210, e.g. by diffusion and/or ion implantation processes.The doped semiconductor region(s) in the semiconductor body 204 of thefirst load terminal structure 220 may include doped source and bodyregions of a vertical power IGFET, for example a superjunction FET or ofa collector of an IGBT, or of an anode or cathode region of a verticalpower semiconductor diode, for example. In the course of processing thesilicon semiconductor body 204 at the first surface 210, depending onthe power semiconductor device to be formed in the semiconductor body, acontrol terminal structure such as a planar gate structure and/or atrench gate structure including gate dielectric(s) and gate electrode(s)may be formed.

The vertical semiconductor device 2001 further includes a second loadterminal structure 225 at a second surface 211, e.g. a rear surface ofthe silicon semiconductor body 204 opposite to the first surface 210.The second load terminal structure 225 includes doped semiconductorregion(s). The doped semiconductor region(s) may be formed by dopingprocesses of the silicon semiconductor body 204 at the second surface211, e.g. by diffusion and/or ion implantation processes. The dopedsemiconductor region(s) in the silicon semiconductor body 204 of thesecond load terminal structure 225 may include doped field stopregion(s), doped drain regions of a vertical power FET, or an emitter ofan IGBT, or an anode or cathode region of a vertical power semiconductordiode, for example.

A first electrical load contact L1 to the first load terminal structure220 and an electrical control terminal contact C to a control terminalstructure, if present in the vertical power semiconductor device, arepart(s) of a wiring area above the first surface 210. A secondelectrical load contact L2 to the second load terminal structure 225 isprovided at the second surface 211. The electrical load contacts L1, L2and the electrical control terminal contact C may be formed of one or aplurality of patterned conductive layers such as metallization layerselectrically isolated by interlevel dielectric layer(s) sandwichedbetween. Contact openings in the interlevel dielectric layer(s) may befilled with conductive material(s) to provide electrical contact betweenthe one or the plurality of patterned conductive layers and/or activearea(s) in the silicon semiconductor body such as the first loadterminal structure 220, for example. The patterned conductive layer(s)and interlevel dielectric layer(s) may form the wiring area above thesemiconductor body 204 at the first surface 210, for example. Aconductive layer, e.g. a metallization layer or metallization layerstack may be provided at the second surface 211, for example.

In the vertical semiconductor device 2001 a current flow direction isbetween the first and second load terminal contacts L1, L2 along avertical direction between the opposite first and second surfaces 210,211.

FIG. 2B is a schematic cross-sectional view of a portion of a lateralsemiconductor device 2002 according to an embodiment. The lateralsemiconductor device 2002 differs from the vertical semiconductor device2001 in that the second load terminal structure 225 and the secondcontact L2 are formed at the first surface 210. The first and secondload terminal structures 220, 225 may be formed simultaneously by sameprocesses. Likewise, the first and second load terminal contacts L1, L2may be formed simultaneously by same processes.

In the embodiments illustrated in FIGS. 2A and 2B, a blocking voltagecapability of the vertical and lateral semiconductor devices 2001, 2002can be adjusted by appropriate distances d₁, d₂ of the drift zone 205between the first and second load terminal structures 220, 225, forexample between a body region and a drain region of a FET.

FIG. 3 is a more detailed a schematic cross-sectional view of oneexample of the vertical semiconductor device 2001 being formed as apower semiconductor diode 2003. The drift zone 205 is n⁻-doped asdescribed in detail with regard to the semiconductor device 2001 above.A p-doped anode region 2201 at the first surface 210 is in electricalcontact with the first load terminal contact L1. The p-doped anoderegion 2201 is an example of an element of the first load terminalstructure 220 illustrated in FIG. 2A. An n⁺-doped cathode region 2251 atthe second surface 211 is in electrical contact with the second loadterminal contact L2. The n⁺-doped cathode region 2251 is an example ofan element of the second load terminal structure 225 illustrated in FIG.2A.

FIG. 4 is a more detailed schematic cross-sectional view of one exampleof the vertical semiconductor device 2001 being formed as a power IGBT2004. The drift zone 205 is n⁻-doped as described in detail with regardto the semiconductor device 2001 above. An emitter structure 2202 at thefirst surface 210 includes a p-doped body region 2203 and an n⁺-dopedsource region 2204. The p-doped body region 2203 and the n⁺-doped sourceregion 2204 are examples of elements of the first load terminalstructure 220 illustrated in FIG. 2A. The emitter structure 2202 is inelectrical contact with the first load terminal contact L1. A gatestructure including a dielectric 240 and a gate electrode 241 is formedon the semiconductor body 205 at the first surface 210. An IGBTcollector including a p⁺-doped rear side emitter 2252 at the secondsurface 211 is in electrical contact to the second load terminal contactL2. The p⁺-doped rear side emitter 2252 is an example of an element ofthe second load terminal structure 225 illustrated in FIG. 2A.

FIG. 5 refers to a method of manufacturing a silicon wafer.

Process feature S100 of the method comprises extracting an n-typesilicon ingot over an extraction time period from a silicon meltcomprising n-type dopants.

Process feature S110 comprises adding p-type dopants to the silicon meltover at least part of the extraction time period, thereby compensatingan n-type doping in the n-type silicon ingot by 20% to 80%.

Process feature S120 comprises slicing the silicon ingot.

In some embodiments, a net n-type doping concentration of the siliconingot is in a range of 1×10¹³ cm⁻³ to 1×10¹⁵ cm⁻³, or in a range of2×10¹³ cm⁻³ to 2×10¹⁴ cm⁻³.

In some embodiments, a segregation coefficient of an n-type dopantspecies of the n-type dopants and a segregation coefficient of a p-typedopant species of the p-type dopants differ by at least a factor ofthree.

In some embodiments, the n-type dopant species is phosphorus and thep-type dopant species is boron.

In some embodiments, the method further comprises adding, in addition toboron, a second p-type dopant species to the silicon melt over at leastpart of the extraction time period, second p-type dopant species havinga segregation coefficient smaller than phosphorus.

In some embodiments, the second p-type dopant species corresponds to atleast one of aluminum and gallium.

In some embodiments, the boron is added to the silicon melt from atleast one of a boron doped quartz material or from boron in a gas phase.

In some embodiments, the boron is added to the silicon melt from a boroncarbide or boron nitride source material.

In some embodiments, the boron is added to the silicon melt from a borondoped crucible.

In some embodiments, the boron doped crucible is formed by at least oneof implanting boron into the crucible, diffusion of boron into thecrucible and in-situ doping.

In some embodiments, the boron is implanted into the crucible at variousenergies and doses.

In some embodiments, the method further comprises applying a thermalbudget to the crucible by heating that is configured to set a retrogradeprofile of the boron in the crucible.

In some embodiments, the method further comprises forming a layer atinner walls of the crucible.

In some embodiments, the method further comprises altering a rate ofadding the boron to the silicon melt.

In some embodiments, altering the rate of adding the boron to thesilicon melt includes altering at least one of size, geometry and rateof delivery of particles, a flow or partial pressure of a boron carriergas.

In some embodiments, altering the rate of adding the boron to thesilicon melt includes at least one of altering a depth of a sourcematerial dipped into the silicon melt and altering a temperature of thesource material, wherein the source material is doped with the boron.

In some embodiments, doping of the source material is carried out by oneof in-situ doping, by a plasma deposition process through a surface ofthe source material, by ion implantation through the surface of thesource material and by a diffusion process through the surface of thesource material.

In some embodiments, the method further comprises controlling a rate ofadding the boron to the silicon melt by measuring a weight of thesilicon ingot during the Czochralski growth process.

In some embodiments, the method further comprises controlling a rate ofadding the boron to the silicon melt by optically measuring a change indimensions of a quartz source material doped with the boron.

In some embodiments, the method further comprises altering a rate ofadding the boron to the silicon melt by altering at least one of acontact area between a source material and the silicon melt and heatingof the source material.

In some embodiments, adding the p-type dopants into the silicon meltincludes dissolving p-type dopants from a p-type dopant source materialinto the silicon melt.

In an embodiment of a CZ silicon ingot, the CZ silicon ingot is dopedwith donors and acceptors and includes an axial gradient of dopingconcentration of the donors and of the acceptors. An electrically activenet doping concentration, which is based on a difference between thedoping concentrations of the donors and acceptors varies by less than60% for at least 40% of an axial length of the CZ silicon ingot due topartial compensation of at least 20% of the doping concentration of thedonors by the acceptors. The electrically active net dopingconcentration may also vary by less than 40%, or by less than 30%, oreven by less than 20% for the at least 40% of the axial length of the CZsilicon ingot. In other words, along at least 40% of the axial length ofthe CZ silicon ingot, the electrically active net doping concentrationmay vary by less than +/−30%, or by less than +/−20%, or by less than+/−15%, or even by less than +/−10% from an average electrically activenet doping concentration averaged along the at least 40% of the axiallength of the CZ silicon ingot. This may be caused by counteractingsegregation effects of donors, which may lead to a strong variation ofnet doping along the axial length of the CZ silicon ingot by means ofpartial compensation with acceptors having another segregation behavior.

In some embodiments, the donors include at least one of phosphorus,arsenic and antimony.

In some embodiments, the acceptors include boron.

In some embodiments, the acceptors further include at least one ofaluminum, gallium and indium.

In some embodiments, a net n-type doping concentration is in a rangefrom 1×10¹³ cm⁻³ to 3×10¹⁴ cm⁻³, or in a range from 2×10¹³ cm⁻³ to2×10¹⁴ cm⁻³.

FIG. 6 is a simplified schematic cross-sectional view of a CZ growthsystem 600 for carrying out the method illustrated in FIG. 5 and formanufacturing a CZ silicon ingot as described in the embodiments above.

The CZ growth system 600 includes a crucible 605, e.g. a quartz crucibleon a crucible support 606, e.g. a graphite susceptor. A heater 607, e.g.a radio frequency (RF) coil surrounds the crucible. The heater 607 maybe arranged at lateral sides and/or at a bottom side of the crucible605. The crucible 605 may be rotated by a supporting shaft 608.

The mixture of silicon material, e.g. a non-crystalline raw materialsuch as polysilicon and an n-type dopant material such as phosphorus(P), antimony (Sb), arsenic (As) or any combination thereof is melted inthe crucible by heating via the heater 607. The n-type dopant materialmay already constitute or be part of the initial doping of the siliconmaterial to be melted and/or may be added as a solid or gaseous dopantsource material. According to an embodiment, the solid dopant sourcematerial is a dopant source particle such as a dopant source pill. Thedopant source material may have a predetermined shape such as a discshape, spherical shape or a cubic shape. By way of example, the shape ofthe dopant source material may be adapted to a supply device 609 such asa dispenser configured to supply the dopant source material to a siliconmelt 610 in the crucible 605.

According to an embodiment, the dopant source material may include, inaddition to the dopant material, a carrier material or a bindermaterial. By way of example, the dopant source material may be quartz orsilicon carbide (SiC) doped with the dopant material. According toanother embodiment, the dopant source material may be a highly dopedsilicon material such as a highly doped polysilicon material that isdoped to a greater extent than the silicon raw material. According toyet another embodiment, the dopant source material may be boron nitrideand/or boron carbide.

A silicon ingot 612 is pulled out of the crucible 605 containing thesilicon melt 610 by dipping a seed crystal 614 into the silicon melt 610which is subsequently slowly withdrawn at a surface temperature of themelt just above the melting point of silicon. The seed crystal 614 is asingle crystalline silicon seed mounted on a seed support 615 rotated bya pull shaft 616. A pulling rate which typically is in a range of a fewmm/min and a temperature profile influence a diameter of the CZ grownsilicon ingot 612.

When extracting the silicon ingot 612 with the CZ growth system 600according to the method illustrated in FIG. 5, boron is added to thesilicon melt 610 over an extraction time period. According to anembodiment, boron is added to the molten silicon at a constant rate. Theboron may be added to the silicon melt 610 from a boron doped quartzmaterial such as a boron doped quartz material supplied to the siliconmelt 610 by the supply device 609. In addition or as an alternative, theboron may be added to the silicon melt 610 from a boron carbide or froma boron nitride source material that may also be supplied to the siliconmelt 610 by the supply device 609.

According to another embodiment, the boron is added to the silicon melt610 from a boron doped crucible. The boron doped crucible may be formedby implanting boron into the crucible, for example (cf. schematiccross-sectional view of FIG. 7). The boron may be implanted into thecrucible 605 by one or more tilted implants, cf. labels I₂ ² and I₃ ²and/or by non-tilted implant, cf. label I₁ ² in FIG. 7. A distributionof tilt angle(s) may be used to adjust the amount of boron that issupplied to the silicon melt 610 by dissolving a material of thecrucible 605 in the silicon melt 610, e.g. at a rate in the range ofapproximately 10 μm/hour in case of a crucible made of quartz. The boronmay be implanted into the crucible at various energies and/or at variousdoses. Applying a thermal budget to the crucible 105 by heating mayallow for setting a retrograde profile of the boron in the crucible 605.Multiple implants at various energies and/or doses further allow forsetting a profile of the boron into a depth of the crucible 605. Thus, arate of adding boron into the silicon melt 610 may be adjusted, i.e. byselection of implantation parameters the rate of the addition of boroncan be varied and controlled in a well-defined manner. By way ofexample, the profile of boron in the crucible 605 may be a retrogradeprofile. As an alternative or in addition to implanting boron into thecrucible 605, boron may also be introduced into the crucible 605 byanother process, e.g. by diffusion from a diffusion source such as asolid diffusion source of boron, for example. As a further alternativeor in addition to the above processes of introducing boron into thecrucible 605, boron may also be introduced into the crucible 605in-situ, i.e. during formation of the crucible 605.

According to yet another embodiment boron may be introduced into thesilicon melt 610 from the gas phase, e.g. by supply of diborane (B₂H₆)via the supply device 609. According to an embodiment, supply of boronin the gas phase may occur via a supply of inert gas into the CZ growthsystem 600. According to another embodiment, supply of boron in the gasphase may occur via one or more tubes, e.g. a quartz tube extending intothe silicon melt 610. According to yet another embodiment, supply ofboron in the gas phase may occur via one or more tubes ending at a shortdistance to the silicon melt 610. The tubes may include one or moreopenings at an outlet, e.g. in the form of a showerhead, for example.

According to another embodiment, a liner layer may be formed on thecrucible 605 for controlling diffusion of boron out of the crucible 605into the silicon melt 610. As an example, the liner layer may be formedof quartz and/or silicon carbide. According to an embodiment, the linerlayer may be dissolved in the silicon melt 610 before boron included inthe crucible gets dissolved in the silicon melt 610 and serves as adopant during the growth process of the silicon ingot 612. This allowsfor adjusting a point of time when boron is available in the siliconmelt as a dopant to be introduced into the silicon ingot 612. The linerlayer may also delay introduction of boron into the silicon melt 610 bya time period that is required for diffusion of boron from the crucible605 through the liner layer and into the silicon melt 610.

According to another embodiment, the method of manufacturing the siliconingot 612 further includes altering a rate of adding the boron to thesilicon melt 610. According to an embodiment, altering the rate ofadding the boron to the silicon melt 610 includes altering at least oneof size, geometry, and rate of delivery of particles including theboron. By way of example, the rate may be increased by increasing adiameter of the particles doped with the dopant material. As anadditional or alternative measure, the rate of adding the boron to thesilicon melt 610 may be increased by increasing a speed of supplying thedopant source material into the silicon melt 610 by the supply device609.

According to another embodiment illustrated in the schematiccross-sectional view of FIG. 8, altering the rate of adding the boron tothe silicon melt 610 includes altering a depth d of a dopant sourcematerial 625 dipped into the silicon melt 610.

According to another embodiment, altering the rate of adding the boronto the silicon melt 610 includes altering a temperature of the dopantsource material 625. By way of example, by increasing a temperature ofthe dopant source material, e.g. by heating, the amount of boronintroduced into the silicon melt 610 out of the dopant source material625 may be increased. The dopant source material 625 is doped with theboron. According to an embodiment, doping of the dopant source materialis carried out by one of in-situ doping, by a plasma deposition processthrough a surface 626 of the dopant source material 625, by ionimplantation through the surface 626 of the dopant source material 625and by a diffusion process through the surface 626 of the dopant sourcematerial 625. The dopant source material 625 may be shaped as a bar, acylinder, a cone or a pyramid, for example. The dopant source material625 may also be made of a plurality of separate dopant source pieceshaving one or a combination of different shapes. The depth d of a partof the dopant source material 625 that is dipped into the silicon melt610 may be changed by a puller mechanism 627. The puller mechanism 627holds the dopant source material 625, dips the dopant source material625 into the silicon melt 610 and also pulls the dopant source material625 out of the silicon melt 610. A control mechanism 128 is configuredto control the puller mechanism 627. The control mechanism 628 maycontrol the puller mechanism 627 by wired or wireless control signaltransmission, for example.

According to another embodiment, altering the rate of adding the boronto the silicon melt 610 includes altering a flow or partial pressure ofa boron carrier gas, e.g. diborane (B₂H₆) when doping the silicon melt610 with boron from the gas phase.

According to an embodiment, the rate of adding the boron to the siliconmelt 610 may be controlled depending on a length of the silicon ingot612 from the seed crystal 614 to the silicon melt 610 during growth.According to another embodiment, the rate of adding the boron to thesilicon melt 610 may be controlled based on a result of measuring aweight of the silicon ingot 612 and/or the dopant source material 625during the Czochralski growth process. By way of example, the weight ofthe silicon ingot 612 and/or the dopant source material 625 may bemeasured by hanging up the silicon ingot 612 and/or the dopant sourcematerial 625 at a weighting unit, for example.

According to an embodiment, boron or another p-type dopant may be addedprior to and/or during CZ growth by a p-dopant source material such as ap-dopant source pill. The p-dopant source material may have apredetermined shape such as a disc shape, spherical shape or a cubicshape. By way of example, the shape of the p-dopant source material maybe adapted to the supply device 609 such as a dispenser configured tosupply the p-dopant source material to a silicon melt 610 in thecrucible 605. A time-dependent supply of a p-dopant into the siliconmelt 610 may be achieved by adjusting a profile of p-type dopantconcentration into a depth of the p-dopant source material, for exampleby multiple ion implantations at different energies and/or by forming aliner layer surrounding the p-dopant source material for controllingdissolving of the p-dopant from the p-dopant source material into thesilicon melt 610 or for controlling the diffusion of the p-dopant out ofthe p-dopant source material into the silicon melt 610.

According to another embodiment, controlling the rate of adding theboron to the silicon melt 610 is carried out by optically measuring achange in dimensions of a quartz source material doped with the boron.Entrance of measurement light into the quartz source material may occurthrough a part of the quartz source material that protrudes from thesilicon melt 610, for example. Altering the rate of adding the boron tothe silicon melt 610 may also be carried out by altering at least one ofa contact area between a dopant source material and the silicon melt andheating of the dopant source material. By altering the rate of addingboron to the silicon melt 610, an effective segregation of boron duringCZ growth can be adapted to the segregation behavior of the n-typedopant(s) so as to achieve an n-type doping partially compensated by 20%to 80% with boron.

According to another embodiment, the net n-type doping is furthercompensated by a p-type dopant species having a segregation coefficientsmaller than phosphorus in addition to boron. According to yet anotherembodiment, the net n-type doping is further compensated by a pluralityof different p-type dopant species having segregation coefficientssmaller than phosphorus. Carrying out partial compensation by boronhaving a segregation coefficient greater than phosphorus and by one ormore p-type dopant species having segregation coefficients smaller thanphosphorus, an effective segregation of p-type dopants during CZ growthcan be adapted to the segregation behavior of phosphorus. This allowsfor a very effective compensation even in the case that source materialis implemented prior to the start of the melting process. Gallium andaluminum are examples of p-type dopant species having a segregationcoefficient smaller than phosphorus. The value of the resultingeffective segregation coefficient can be adjusted by the ratio betweenthe p-type dopant species with higher segregation coefficient and thep-type dopant species with lower segregation coefficient. Typically, theratio between B and Al or Ga is at least 2, or even higher than 5 oreven higher than ten for the case of phosphorus doping.

The method for manufacturing the silicon ingot 112 described aboveincludes a partial compensation where donors in the n-doped siliconingot 112 outnumber boron and optional further p-type dopants that areadded to the silicon melt 110 during CZ growth.

An axial profile of doping caused by segregation of dopant materialduring CZ growth can be approximated by equation (1) below:

$\begin{matrix}{{c(p)} = {{k_{0}{c_{0}\left( {1 - p} \right)}^{k_{0} - 1}} + {F_{0}{\frac{k_{0}}{1 - k_{0}}\left\lbrack {\left( {1 - p} \right)^{k_{0} - 1} - 1} \right\rbrack}}}} & (1)\end{matrix}$

The first term in the equation (1) refers to a doping that has beenadded to the melt before extracting the silicon ingot from the melt.According to the above embodiments, n-type dopant materials may bedescribed by the first term of equation (1). The second term refers toadding dopant material at a constant rate into the melt during CZgrowth. According to the above embodiments, adding the boron may bedescribed by the second term of equation (1).

In the above equation (1), c(p) denotes a concentration of the dopantmaterial in the silicon ingot (atoms/cm³), p denotes a portion of theinitial melt during CZ growth that has been crystallized and correspondsto an axial position between 0% and 100% of the completely grown siliconingot, k₀ denotes a segregation coefficient of the dopant material, e.g.approx. 0.8 for boron (B) in silicon and approx. 0.35 for phosphorus (P)in silicon, c₀ denotes an initial concentration of the dopant materialin the melt (atoms/cm³) and F₀ denotes a total amount of the dopantmaterial that is constantly (relative to the pulling rate) added to themelt divided by the initial volume of the melt (atoms/cm³).

FIG. 9 illustrates calculated concentrations of non-compensatedphosphorus (P), i.e. net n-doping versus an axial position betweenopposite ends of a silicon ingot. The illustrated curves refer todifferent ratios of boron (B) and phosphorus (P), i.e. F_(0B)/c_(0P)corresponding to the ratio of the total amount of boron that isconstantly (relative to the pulling rate) added to the silicon meltdivided by the initial volume of the melt (F_(0B) in atoms/cm³) and aninitial concentration of phosphorus in the melt (c_(0P) in atoms/cm³).

The illustrated curves refer to values of F_(0B)/c_(0P) of 0%, 10% ,20%, 30%, 40%, 50%. By adding boron to the melt during CZ growth andthereby adding a compensation dopant to the melt during the CZ growth,the method described with reference to FIGS. 5 to 8 allows for siliconwafers suitable for manufacturing semiconductor devices having improvedrobustness and reliability. When adding the boron to the melt beforeinitiating CZ growth of the silicon ingot, homogeneity of the netn-doping concentration along the axial direction between opposite endsof the silicon ingot may be even worse than for the case ofF_(0B)/c_(0P) of 0%, i.e. without adding boron. This is due to thelarger segregation coefficient of the compensation dopant boron comparedto the segregation coefficient of the n-type dopant such as phosphorus.By partial compensation of at least 20% of P by B, an electricallyactive net doping concentration, which is based on a difference betweenthe doping concentrations of the donors and acceptors varies by lessthan 60% from an average value for at least 40% of an axial length ofthe CZ silicon ingot. Variation may be even kept smaller by an optimizedcounteraction to segregation effects of the donors by compensation withacceptors having a different segregation behavior. Thereby, theelectrically active net doping concentration may also vary by less than40%, or less than 30%, or even less than 20% for the at least 40% of theaxial length of the CZ silicon ingot.

FIG. 10 illustrates calculated specific resistance curves versus anaxial position between opposite ends of a silicon ingot. Similar to theparameter curves illustrated in FIG. 9, the curves illustrated in FIG.10 refer to different ratios of boron (B) and phosphorus (P), i.e.F_(0B)/c_(0P) corresponding to the ratio of the total amount of boronthat is constantly (relative to the pulling rate) added to the siliconmelt divided by the initial volume of the melt (F_(0B) in atoms/cm³) andan initial concentration of phosphorus in the melt (c_(0P) inatoms/cm³).

Similar to the parameter curves illustrated in FIG. 9, the curvesillustrated in FIG. 10 refer to values of F_(0B)/c_(0P) of 0%, 10% ,20%, 30%, 40%, 50%. By adding boron to the melt during CZ growth andthereby adding a compensation dopant to the melt during the CZ growth,the method described with reference to FIGS. 5 to 8 allows for improvinghomogeneity of the specific resistance along the axial direction betweenopposite ends of the silicon ingot and for silicon wafers suitable formanufacturing semiconductor devices having improved robustness andreliability.

Based on the method illustrated and described with respect to FIGS. 5 to10, table 1 illustrates a maximum portion of the ingot along the axialdirection having a specific fluctuation of specific resistance and aspecific ratio of boron (B) and phosphorus (P), i.e. F_(0B)/c_(0P)corresponding to the ratio of the total amount of boron that isconstantly (relative to the pulling rate) added to the silicon meltdivided by the initial volume of the melt (F_(0B) in atoms/cm³) and aninitial concentration of phosphorus in the melt (c_(0P) in atoms/cm³).Table 1 refers to values of F_(0B)/c_(0P) of 0%, 10% , 20%, 30%, 40%,50%, and to axial fluctuations of the specific resistance of +/−5%,+/−10%, +/−15%, +/−20%, +/−30%, +/−50%. By adding boron to the meltduring CZ growth and thereby adding a compensation dopant to the meltduring the CZ growth, the method described with reference to FIGS. 4 to10 allows for a yield improvement by increasing the maximum portion ofthe ingot along the axial direction having a specific fluctuation ofspecific resistance. As an example, the axial portion of the ingothaving a fluctuation of specific resistance of +/−10% may be increasedfrom 26% (no compensation doping) to 78% (compensation dopingF_(0B)/c_(0P) of 40%).

TABLE 1 boron compensation flow/initial maximum ingot length with axialdoping with fluctuation of specific resistance of phosphorous +/−5%+/−10% +/−15% +/−20% +/−30% +/−50% no 14% 26% 36% 46% 60% 80%compensation 20% 32% 48% 58% 66% 76% 88% 30% 56% 66% 74% 78% 84% 92% 35%66% 74% 78% 82% 86% 92% 40% 38% 78% 82% 84% 88% 92% 45% 22% 44% 84% 86%88% 94%

According to the method illustrated with respect to FIGS. 9 to 10, boronis constantly added (relative to the pulling rate) to the silicon melt(described by the term F_(0B) in atoms/cm³) and phosphorus is added asan initial concentration to the melt (described by the term c_(0P) inatoms/cm³). According to other embodiments, boron may be added to themelt at an altering rate. Apart from or in addition to phosphorus, othern-type dopant materials such as antimony or arsenic may be used.

In addition to adding boron to the melt during CZ growth a part of theoverall boron may also be added to the melt before CZ growth which maybe described by a term c_(0P) in equation (1). Likewise, in addition toadding phosphorus or another n-type dopant material as an initialconcentration to the melt, a part of the phosphorus or the other n-typedopant may also be added to the melt during CZ growth which may bedescribed by a term F_(0P) in equation (1) in case of constantly addingthe phosphorus or the other n-type dopant material relative to thepulling rate.

Slicing of the silicon ingot into silicon wafers may be carried outperpendicular to a central growth axis of the silicon ingot. Accordingto an embodiment, slicing is carried out by an appropriate slicing toolsuch as an inner-diameter (ID) saw or a wire type saw, for example.

The embodiments described herein may be combined. Although specificembodiments have been illustrated and described herein, it will beappreciated by those of ordinary skill in the art that a variety ofalternate and/or equivalent implementations may be substituted for thespecific embodiments shown and described without departing from thescope of the present invention. This application is intended to coverany adaptations or variations of the specific embodiments discussedherein. Therefore, it is intended that this invention be limited only bythe claims and the equivalents thereof.

What is claimed is:
 1. A CZ silicon ingot comprising donors andacceptors and having an axial gradient of doping concentration of thedonors and of the acceptors, wherein at least 20% of the dopingconcentration of the donors is compensated by the acceptors so that anelectrically active net doping concentration of the CZ silicon ingot,which is based on a difference between the doping concentrations of thedonors and acceptors, varies by less than 60% for at least 40% of anaxial length of the CZ silicon ingot.
 2. The CZ silicon ingot of claim1, wherein the electrically active net doping concentration varies byless than 20% of the electrically active net doping concentration forthe at least 40% of the axial length of the CZ silicon ingot.
 3. The CZsilicon ingot of claim 1, wherein the donors include at least one ofphosphorus, arsenic and antimony.
 4. The CZ silicon ingot of claim 1,wherein the acceptors include boron.
 5. The CZ silicon ingot of claim 1,wherein the acceptors further include at least one of aluminum, galliumand indium.
 6. The CZ silicon ingot of claim 1, wherein a net n-typedoping concentration of the CZ silicon ingot is in a range from 1×10¹³cm⁻³ to 3×10¹⁴ cm⁻³.